Field of the Invention
The present invention relates to a light amount adjusting device, a lens barrel, and an image capturing apparatus, and more particularly, to those suitable for adjustment of an incoming light amount.
Description of the Related Art
A camera including a neutral density (ND) filter inside a lens barrel is known as one example of an optical apparatus including a light amount adjusting device for acquiring an appropriate exposure amount.
The light amount adjusting device inside the lens barrel is often disposed near a diaphragm. The light amount adjusting device includes an ND filter formed in size for covering the entire diameter of an aperture through which light flux passes. A general ND filter is movably supported between a position in which the ND filter completely covers the aperture and a position in which the ND filter is completely retracted from the aperture. Accordingly, a general light amount adjusting device has an outer diameter that needs to be at least three times as large as the diameter of the aperture in consideration of space in which the retracted ND filter is stored.
Moreover, a light amount adjusting device including a plurality of ND filters is known. An optical apparatus discussed in each of Japanese Patent Application Laid-Open No. 2007-292828 and Japanese Patent Application Laid-Open No. 2011-248029 includes two ND filters. Each of the two ND filters includes a transparent portion having a uniform transmittance and a gradation portion having a transmittance that continuously changes. When an image of a high-brightness subject is captured, the two ND filters are arranged to cover the aperture. This attenuates the light to acquire an appropriate light amount. Here, an amount of the gradation portion to be inserted into the aperture changes depending on a position of each of the ND filters, and a combined density acquired by overlaying the two filters changes. Consequently, adjustment of a position of each of the ND filters enables a light amount to be adjusted in a non-stepwise manner.
Although cameras of recent years are expected to be compact, installation of a large image capturing element and an optical element with a small F number (i.e., an optical system having a large aperture diameter) is demanded. That is, the camera is demanded that a lens barrel is smaller and an aperture diameter covered with an ND filter is larger.
However, a light amount adjusting device including a single ND filter has an outer diameter which needs to be at least three times as large as the aperture diameter, as described above. Thus, an increase in the aperture diameter increases size of the light amount adjusting device. This causes difficulty in reducing size of the camera.
Since the light amount adjusting device discussed in Japanese Patent Application Laid-Open No. 2007-292828 can adjust a combined density of the two ND filters in a non-stepwise manner, a width of the gradation portion is greater. This increase size of the light amount adjusting device.
In the light amount adjusting device discussed in Japanese Patent Application Laid-Open No. 2011-248029, each of the two ND filters includes the transparent portion. At the time of retraction, the ND filter including the transparent portion needs to be retracted from the aperture. This increases space for storing the retracted ND filter. Consequently, size of the light amount adjusting device is increased.